Separation and Purification Technology 98 (2012) 102–108

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Separation and Purification Technology

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Electrochemical dosing of iron and aluminum in continuous processes: A key stepto explain electro-coagulation processes

Carlos Jiménez, Cristina Sáez ⇑, Fabiola Martínez, Pablo Cañizares, Manuel A. RodrigoDepartment of Chemical Engineering, University of Castilla La Mancha, Campus Universitario s/n, 13071 Ciudad Real, Spain

a r t i c l e i n f o

Article history:Received 23 December 2011Received in revised form 5 July 2012Accepted 5 July 2012Available online 16 July 2012

Keywords:Electro-coagulationIronAluminumSpeciationZ-potential

1383-5866/$ - see front matter � 2012 Elsevier B.V. Ahttp://dx.doi.org/10.1016/j.seppur.2012.07.005

⇑ Corresponding author. Tel.: +34 9902204100x670E-mail address: cristina.saez@uclm.es (C. Sáez).

a b s t r a c t

The aim of this work is to study the main differences between aluminum and iron production during elec-trochemical coagulation processes, attending not only to the coagulant dosage, but also to the character-istics of the coagulants produced. To do this, the influence of both pH and current density have beenstudied. It has been observed that there is a super-faradaic production of aluminum because of a pro-moted corrosion at the alkaline pHs produced on the cathode surface. This efficiency decreases for highvalues of current density because of cathodic protection processes. On the contrary, for iron electrodes, itdoes not exist a super-faradaic dissolution within the range of current densities studied but simply adecrease in the efficiency for alkaline pH values and high current densities, because of the formationof Fe3+ instead of Fe2+ during the electro-dissolution process. With respect to speciation, it has beenobserved the formation of monomeric species at acidic pHs and precipitate species for pHs above 5. Inaddition, for aluminum, monomeric species are also the primary species for pHs above 9 and some poly-meric species are formed at pHs between 4 and 6. The superficial charge of both metal precipitates is verysimilar. For pHs below 9, both aluminum and iron precipitates have a positive charge due to the adsorp-tion of monomeric soluble hydroxometals positively charged or protons on the precipitate surface. ForpHs above 9 the aluminum precipitate is negatively charged due to the adsorption of soluble hydroxo-metal species negatively charged or hydroxyl ions on the precipitate surface. On the contrary, iron pre-cipitate does not have a superficial charge for alkaline pH, because there is almost no formation of solublenegatively-charged species.

� 2012 Elsevier B.V. All rights reserved.

1. Introduction

For the study of water or wastewater coagulation through elec-trochemical production of metallic ions (technology commonlyknown as electro-coagulation), it is very important to know theproduction rate and the chemical species which are produced oncethe electrode starts dissolving, and how they behave in solution.

As these processes are not limited by mass transport (contraryto many other electrochemical environmental remediation pro-cesses, because the reagent to be oxidized is the electrode itself),Butler–Voltmer equation is the law which governs the metal disso-lution of the anode when a potential difference is applied to theelectrochemical cell.

When aluminum or iron are used as electrodes, release of Al3+

or Fe2+ ions is known to be produced according to Eqs. (1) and(2). However, different studies have demonstrated the formationof higher concentrations of aluminum than those expected fromthe theoretical values predicted by Faraday’s Law for the electrol-ysis of metal sheets [1,2] as a consequence of the corrosion of

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the electrodes by the reactive media, especially under the presenceof chloride ions. On the contrary, for iron, the electro-dissolutionefficiency uses to be around 100% or slightly below, according tothe literature [1].

A1! A13þ þ 3e� ð1Þ

Fe! Fe2þ þ 2e� ð2Þ

Electrochemically-produced ions can turn into different speciesdepending of the pH of reactive media. In addition, more signifi-cant changes occur for iron, as it is further oxidized in solutiononce produced even by oxygen. At this respect, it is known thatFe2+ oxidizes very slowly in contact with dissolved oxygen toFe3+ in acidic media, according with Eq. (3). In neutral or alkalinemedia, Fe2+ is immediately transformed into its hydroxide, whichis quickly oxidized by dissolved oxygen to iron hydroxide (III)(Eqs. (4) and (5)).

2Fe2þ þ 1=2O2 þH2O2 ! Fe3þ þ 2OH� ð3Þ

Fe2þ þ 2OH� ! FeðOHÞ2 ð4Þ

C. Jiménez et al. / Separation and Purification Technology 98 (2012) 102–108 103

2FeðOHÞ2 þ 1=2O2 þH2O! 2FeðOHÞ3 ð5Þ

The complexity of aluminum and iron chemistry is widely de-scribed in the literatures [3–7]. According to these studies, electro-chemically-produced metallic-ions can be hydrolyzed close to theanode to form reaction intermediates (positive or negativelycharged soluble hydroxocomplexes or metal insoluble hydroxides)very active in destabilization of dispersed or dissolved particles inwastewaters according to Eqs. (6)–(10). Table 1 shows the equilib-rium constants for aluminum [8] and iron [9]. Moreover, it isknown the formation of polymers from the coupling of these spe-cies including intermediate molecular-weight species (oligomers)and high molecular-weight species (polymers). According to theliteratures [5,7], the main parameters that affect the speciationare total metal concentration, pH, [OH�]/[Me] ratio and the natureof the electrolyte.

Meþ3 þH2O ¢ MeðOHÞþ2 þHþ K1 ð6Þ

MeðOHÞþ2 þH2O ¢ MeðOHÞþ2 þHþ K2 ð7Þ

MeðOHÞþ2 þH2O ¢ MeðOHÞ3 þHþ K3 ð8Þ

MeðOHÞ3 þH2O ¢ MeðOHÞ�4 þHþ K4 ð9Þ

MeðOHÞ3 þH2O ¢ Me3þ þ 3OH�Ks ð10Þ

At this point, it is important to notice that the differences in thechemistry of aluminum and iron can affect the coagulation mech-anisms for the removal of pollutants from waters or wastewaters.

Hence, and according to this background, the objective of thiswork is to analyze the production rate and the characteristics ofthe coagulants produced from the oxidation of aluminum and ironsheets in continuous electro-coagulation reactors operating at typ-ical conditions, as a function of current density and pH. To achievethis objective three aspects of high importance have been studied:the amount of coagulation reagent produced in the reactor, the sortof hydrolysis species generated in reactive media and the superfi-cial charge of the aluminum and iron flocs formed. This knowledgeis very useful to understand the coagulation mechanisms that oc-cur inside the electrochemical cell during coagulation processesfor both electrode materials. With this information, the optimalelectrode material can be selected as a function of the desiredcoagulation mechanism.

2. Experimental

2.1. Electro-coagulation setup

The electrochemical experiments made to determine the alumi-num and iron concentration and their hydrolysis species as a func-tion of current density and pH have been carried out in a bench-scale plant, with a single-compartment electrochemical flow cell(described elsewhere [10,11]). Aluminum or iron electrodes wereused as the anode and cathode. Both electrodes were square inshape with a geometric area of 100 cm2 each and with an electrode

Table 1Hydrolysis and solubility constants of aluminum andiron ions in aqueous solution in zero ionic force and25 �C.

Constant Al Fe

pK1 4.9 2.2pK2 5.6 3.5pK3 6.7 6.0pK4 5.6 10.0pKS 31.5 38.0

gap of 9 mm. In every experiment the anodic and cathodic materi-als were the same. This is a normal practice in industrial electro-coagulation processes, because this allows the inversion of thepolarity as a response to avoid operation problems, which can becaused by the formation of films of carbonates on the surface ofthe cathodes, or by the passivation of the anodes. The electricalcurrent was applied using a dc power supply, PROMAX FA-376.The current flowing through the cell was measured with a Keithley2000 digital multimeter. The electrolyte was stored in a 5000 mLglass tank, stirred by an overhead stainless steel rod stirrer, Hei-dolph RZR 2041, thermostated by means of a water bath to main-tain the temperature at the desired set point, and circulatedthrough the electrolytic cell by a peristaltic pump.

2.2. Characterization of aluminum and iron speciation

The characterization of the hydrolyzed aluminum or iron speciesgenerated has been carried out by the well-known ferron method[12–18]. This method consists of the timed spectroscopy monitor-ing of metal (aluminum or iron) ferron (8-hydroxy-7-iodo-5-quin-olinesulfonic acid) reaction, to form a complex of probablecomposition [14] Me(ferron)3 which has a maximum absorbanceof 364 nm. Monomeric species react almost instantaneously withferron, whereas polymeric species have a much slower reaction ratewith this compound. The particles of precipitate practically do notreact with ferron. Therefore, this method allows distinguishingamong monomeric, polymeric or precipitate species.

The analytical measurement has been carried out by filteringthe samples using micropore membranes of 0.45 lm to removethe particles of precipitate. Once the sample is filtered, an aliquotis added to the volume of saturated ferron solution freshly pre-pared (as ferron is not stable [16]) so that ferron is in excess, atpH 5 in an acetate buffered solution. Immediately, the absorbanceof the sample is monitored with time, until a constant value is ob-tained, what is indicative of the end of the reaction.

By plotting the logarithm of the unreacted metal (aluminum oriron) vs. time, the ratio of hydrolyzing-metal species that reactquickly and slowly with ferron (that are, monomeric and polymericspecies) can be estimated. Extrapolation of the linear parts of thecurve to zero time yields information on the amount of metal thatis bound in complexes of different degree of polymerization [12–14]. The measurement of total and soluble metal (filtered with0.45 lm) reports the ratio of soluble and precipitate metal.

2.3. Experimental procedure

To determine the amount of aluminum and iron electro-pro-duced at the different operation conditions, and the hydrolysis spe-cies formed, several experiments were carried out. In these assays,the electrolyte consists of NaCl, and NaOH or HCl added for anysubsequent pH adjustment. After the experiments, samples weremonitored by ferron method, and aluminum or iron concentration,zeta potential and pH were then measured off-line. The aluminumand iron concentrations were determined by dilution 50:50 v/v ofsamples with HNO3 4 N, and measured using an Inductively Cou-pled Plasma LIBERTY SEQUENTIAL VARIAN according to a standardmethod [19] (Plasma Emission Spectroscopy). Zeta Potential wasmeasured using a Zetasizer Nano ZS (Malvern, UK).

3. Results and discussion

3.1. Electro-dissolution of aluminum and iron electrodes

To study the differences in the metal electro-dissolution effi-ciency (when using aluminum or iron sheets as electrode materialsin electro-coagulation reactors), the metal concentration dosed at

104 C. Jiménez et al. / Separation and Purification Technology 98 (2012) 102–108

the steady-state to a particular flow of an aqueous solution (simu-lating the water to be electro-coagulated) has been evaluated as afunction of current density and pH, during continuous electro-coagulation processes. Both parameters have demonstrated in pre-vious studies that were of great importance in the dissolution effi-ciency of the metal sheets used as electrodes [18]. Duringexperiments, ratio (water flow)/(electrode area) was kept constantat a typical value of 1.9 m/h.

Fig. 1 shows the influence of pH and current density on the totalaluminum concentration dosed at steady state. In this figure, the-oretical values expected according to Faraday’s Law are also repre-sented. First of all, it can be observed that in every case aluminumconcentration is above Faraday’s Law expected values. This behav-ior has been explained in previous works [2] in terms of the corro-sion of the aluminum sheets at alkaline pHs in chloride media. So,the alkaline pH achieved in the cathode of the reactor due to waterreduction would favor the corrosion of aluminum sheets increasingthe aluminum concentration above the theoretical values. In addi-tion, it can be seen that for pHs values between 5 and 7, aluminumdissolution efficiency decreases. Moreover, within this pH range anincrease in experimental cell voltage (necessary to achieve the cur-rent density fixed, as the experiments work under galvanostaticconditions) was observed (data not shown). For pHs above 7, alu-minum concentration increases again and cell voltage decreases.These results can be explained in terms of the passivation of alumi-num with a layer of hydrargilite (Al2O3�3H2O), which it is known tooccur for pHs around 4 and 8.5, according to Pourbaix diagram[20]. Limits of the passivity zone depend of temperature and thestructure of the oxide layer, which depends of the room conditionsthat exists during its formation. Beyond these limits, the corrosionof the electrodes at acidic and, especially, at alkaline pHs, increasesthe aluminum dissolution efficiency. When current density in-creases, it can be seen that the aluminum concentration increaseslinearly for lowest current densities and independently of pH.However, when current density increases above 20 mA cm�2 thealuminum dissolution efficiency decreases. This behavior can beexplained attending to the promotion of water oxidation that com-petes with aluminum oxidation Eq. (11) especially for high currentdensity values. This behavior could be also related to a decrease ofthe extension of the corrosion process when increasing currentdensity. This process is known as cathodic protection, and it woulddecrease the aluminum dissolution in the cathode due to thecorrosion of the electrodes.

0

1

2

3

4

5

6

0 5 20 30 50

current density / mA cm-2

Al d

osed

/ m

mol

Al d

m-3 pH 2

pH 5

pH 7

pH 9

pH 12

faraday

Fig. 1. Variation of aluminum concentration in the electrochemical cell as afunction of current density for different values of steady-state pH. Temperature:25 �C; flow: 19 dm3 h�1; supporting electrolyte: 3000 mg dm�3 NaCl; flow rate:19 dm�3 h�1. — Theoretical values predicted by Faraday’s Law.

2H2O� 4e� ! O2 þ 4Hþ ð11Þ

Same study has been made with iron electrodes. Fig. 2 showsthe variation of iron concentration at steady-state conditions withcurrent density for different pH values. Theoretically, iron oxida-tion leads to the formation of Fe(II) ions. However, for the analysisof the results, theoretical values predicted by Faraday’s Law havebeen represented taking into account the formation of Fe(II) ions(continuous line) and Fe(III) ions (discontinuous line). It can beseen that iron increases linearly with current density and, contraryto aluminum, it does not achieve dissolution efficiencies above Far-aday’s Law. This is because of iron corrosion occurs merely foracidic pHs and in a much lower extent that for aluminum. How-ever, it can be seen that when pH increases above 8, dissolutionefficiency decreases below Faraday’s Law values for Fe(II), andexperimental values are closer to the theoretical formation valuesfor Fe(III). This fact can be explained taking into account that foracidic pHs and low electrode potentials (those achieved for lowcurrent densities), iron oxidation leads to Fe(II) formation, accord-ing to iron Pourbaix diagram [21]. However, for alkaline pHs, ironoxidation leads to Fe(III) formation as oxide or hydroxide. Thismakes decrease the iron concentration produced, because Fe(III)formation requires 3 electrons instead of 2 and so, a higher currentvalue for achieving the same iron concentration. Same effect occursfor acidic pHs and high electrode potential. In those conditions,Fe(III) is generated from iron oxidation. This can explain that forlowest current densities (for neutral and acidic pHs), experimentalresults are very close to Fe(II) theoretical formation values. How-ever, when current density increases (and so, cell potential andelectrode potential) the experimental values are below Fe(II) theo-retical values and closer to Fe(III) formation, because in those con-ditions both iron ions can be formed simultaneously from ironsheets inside the electrochemical cell.

3.2. Aluminum and iron speciation

Fig. 3 shows the ratio between the different aluminum hydroly-sis species produced in the electrochemical cell as a function of pH,for the different current densities studied in this work. Hydrolysisspecies has been divided into monomeric, polymeric and precipi-tates, according to the characterization technique used [16].

0

1

2

3

4

5

6

0 5 20 30 50

current density / mA cm-2

Fe

dose

d / m

mol

Fe

dm-3

pH 2

pH 5

pH 7

pH 9

pH 12

faraday Fe 2+

faraday Fe 3+

Fig. 2. Variation of iron concentration in the electrochemical cell as a function ofcurrent density for different values of steady-state pH. Approximate values ofsteady-state pH. Temperature: 25 �C; flow: 19 dm3 h�1; supporting electrolyte:3000 mg dm�3 NaCl; flow rate: 19 dm�3 h�1. – Theoretical values predicted byFaraday’s Law for Fe2+ generation; — theoretical values predicted by Faraday’s Lawfor Fe3+ generation.

0

20

40

60

80

100

1 3 5 7 9 11 13

pH

Al/

%

0

20

40

60

80

100

1 3 5 7 9 11 13

pH

Al/

%

0

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100

1 3 5 7 9 11 13

pH

Al/

%

0

20

40

60

80

100

1 3 5 7 9 11 13

pH

Al/

%

(a) (b)

(c) (d)

Fig. 3. Influence of the pH on the aluminum species formed in electrochemical continuous experiments for different current densities: (a) 5 mA cm�2; (b) 20 mA cm�2; (c)30 mA cm�2; (d) 50 mA cm�2. Temperature: 25 �C; supporting electrolyte: 3000 mg dm�3 NaCl; flow rate: 19 dm3 h�1: ��e�� monomeric hydroxoaluminum ions; –x–polymeric hydroxoaluminum ions; –�– aluminum hydroxide precipitates (all expressed as percentages of the total aluminum concentration).

0

20

40

60

80

100

1 3 5 7 9 11 13

pH

Fe/ %

0

20

40

60

80

100

1 3 5 7 9 11 13

pH

Fe/ %

0

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40

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1 3 5 7 9 11 13

pH

Fe/ %

0

20

40

60

80

100

1 3 5 7 9 11 13

pH

Fe/ %

(a) (b)

(c) (d)

Fig. 4. Influence of the pH on the iron species formed in electrochemical continuous experiments for different current densities: (a) 5 mA cm�2; (b) 20 mA cm�2; (c)30 mA cm�2; (d) 50 mA cm�2. Temperature: 25 �C; supporting electrolyte: 3000 mg dm�3 NaCl; flow rate: 19 dm3 h�1: ��e��, monomeric hydroxoiron ions; –x–,polymeric hydroxoiron ions; –�–, iron hydroxide precipitates (all expressed as percentages of the total iron concentration).

C. Jiménez et al. / Separation and Purification Technology 98 (2012) 102–108 105

As it can be seen, aluminum speciation changes significantlywith both pH and current density. For acidic pHs, monomerichydrolysis species are mainly formed, with a very small amountof precipitate and polymeric species. For pHs around neutrality(between 5 and 9), aluminum hydroxide precipitates are the pri-mary species, with very few amounts of soluble species for thelowest current density. From pH 9 the precipitate formation de-creases, and increases monomeric and polymeric aluminum ratios.The highest proportion of polymeric aluminum species are formedfor pHs between 4 and 5 in every case. As a function of current

density, it can be seen that the lowest the current density, the nar-rowest the precipitate formation zone is. The amplitude of thecurve increases with current density up to achieve a steady-statefor current densities above 20 mA cm�2. From that point, the ratioof the different aluminum hydrolysis species remains unchanged.

In the same way, Fig. 4 shows the variation of iron speciationwith pH and current density. First of all, it can be observed thatwhen using iron electrodes the main hydrolysis species generatedare soluble monomeric species for acidic pHs, and precipitate foralkaline pHs. Precipitate ratio increases with pH up to a 100% of

-40-30-20-10

0

10203040

0 1 2 3 4 5 6 7 8 9 10 11 12 13

pH

Z p

oten

tial

/ mV

-40-30-20

-100

1020

3040

0 1 2 3 4 5 6 7 8 9 10 11 12 13

pH

Z p

oten

tial

/ mV

(a)

(b)

Fig. 5. Zeta potential variation of aluminum (a) and iron (b) precipitates formedduring electrochemical assays as a function of the pH for different current densities:� 5 mA cm�2; e 20 mA cm�2; D 30 mA cm�2; x 50 mA cm�2.

106 C. Jiménez et al. / Separation and Purification Technology 98 (2012) 102–108

iron hydroxide precipitate for pH values above 7. In the same way,monomeric iron ratio decreases when pH increases. With iron, al-most no polymeric species are formed (less than 1% for very acidicpH). In this case, it can be seen that as current density increasesand so, total iron concentration, there is almost no variation ofthe species formed in the reaction media. Nevertheless, it can beobserved a slight change in the precipitate formation curvethrough more alkaline pH values. This behavior is opposite to theexpected, because theoretically when increasing current density,

Fig. 6. Scheme of the adsorption of soluble charged species above the me

pH values for precipitate formation should decrease as occurs foraluminum.

3.3. Z potential variation of aluminum and iron flocs

Once the aluminum and iron dosing rate has been studied as afunction of current density and pH, and their hydrolysis specieshave been characterized, it is worth to discuss changes occurringon the surface of aluminum and iron flocs during the electrochem-ical dosing. The superficial charge of these flocs is of a great impor-tance for electro-coagulation processes as many pollutantscontained in waters or wastewaters are charged-species and, con-sequently, the success of an electro-coagulation process could bemuch related to electric interactions between reagents and pollu-tants. As an example, a precipitate with the opposite charge couldfavor the entrapment of the colloidal particles of wastewaterswhile a precipitate with the same charge could lead to a very inef-ficient process.

Fig. 5 shows the Z potential of metal flocs formed during theelectro-dissolution of aluminum and iron sheets in electro-coagu-lation reactors as a function of the pH, for different values of cur-rent density. It can be observed a similar behavior for bothaluminum and iron: the superficial charge of the aluminum or ironprecipitate does not depend on the current density, but just on thepH. For pHs below 8–9, the surface of aluminum or iron flocs ispositively charged. From pH 9, charge reverses for aluminum andget closer to the isoelectric point for iron.

Attending to these results, and taking into account speciation ofiron and aluminum, the occurrence of both positive and negativelycharged particles can be explained in terms of the adsorption ofsoluble monomeric species on the metal hydroxide precipitateformed. So, for acidic pH different aluminum or iron monomericspecies as MðOHÞþ2 , M(OH)2+, M3+ (M = Al or Fe) or protons willbe adsorbed according to the scheme showed in Fig. 6, whereasfor aluminum at alkaline pHs both monomeric negative speciesas AlðOHÞ�4 or hydroxyl ions would be adsorbed on the precipitate.

tal precipitate. (a) Aluminum (left: pH < 9 and right: pH > 9). (b) Iron.

pH41210184 620

Log

[M

0

-2

-4

-6

-8

-10

-12

Fe(OH)3

Al(OH)3

pH41210184 620

Log

[M

x(O

H) y

3x-y

] / m

ol d

m-3

0

-2

-4

-6

-8

-10

-12

FFFeeeeee(OH(OH(OH)))333333

AlAlAlAl(((((OHOHOHOHH)))))3333

Fe(OH)3

Al(OH)3

Fig. 7. Species concentration diagram for iron and aluminum. Only precipitateformation zone is represented.

C. Jiménez et al. / Separation and Purification Technology 98 (2012) 102–108 107

Taking into account the characteristics of the reagent speciesproduced during electro-coagulation, it can be proposed some rec-ommendations about the application of iron or aluminum electro-coagulation for different pollutants, in order to promote the re-moval of this particular pollutant and, at the same time, to savecoagulation reagents. At this point, it is worth to state that dueto the lower solubility (Fig. 7), iron electro-coagulation would bethe recommended technology if floc enmeshment is looked for,but this coagulation mechanism implies the use of huge amountsof reagent and hence higher costs in terms of reagents and sludgetreatment. In addition the higher atomic weight of iron makes thatfor the same charge dosage, the amount of sludge produced withiron would be greater.

At this point, for the removal of anions, although both, iron andaluminum, can produce positively charge precipitates, it is betterto use iron electro-coagulation (at pHs close to 7 but even till 8) be-cause of the larger mass of iron as compared to aluminum for thesame molarity (and consequently for the same charge), and alsobecause of the positive charge of the iron precipitate at these con-ditions which favors adsorption of anions [22]. On the contrary forthe removal of cations, aluminum electro-coagulation at slightlyalkaline pHs is the better choice, particularly at pH above 8–9, be-cause of the negative charge of the precipitates which favors thestrong adsorption of cationic species [22] on their surface.

In the case of colloids [23], those negatively charged can be bet-ter removed by neutralization with aluminum hydroxo-cations atacidic pHs and, at closer to neutrality pHs by surface chargeenmeshment. Iron can also have good efficiencies, but it will leadto a higher production of sludge, because of the bigger mass addedof this reagent for the same charge neutralization (atomic weightof aluminum is less than half of the atomic weight of iron). Thesame can be said for positively-charged colloids, although in thiscase slightly alkaline pHs, within the range 8–9, would favor theremoval. If charge in the colloids is not very important, the highermass of iron (for the same molar dosage) advice its use. A last caseis the removal of o/w emulsions [23], which is very similar to col-loids suspensions. If they are stabilized by anionic surfactants theybehave as negatively charged colloids, and if they are stabilized bycationic surfactants they behave as cationic colloids.

4. Conclusions

The main conclusions obtained from this work have been:

– The metal dissolution process in an electrochemical cell is dueto both electrochemical and corrosion processes. So, thealuminum dissolved in the cell is higher than the expected

values predicted by Faraday’s Law. In the case of iron, on thecontrary, dissolution efficiency is lower than 100% if Fe2+ forma-tion is supposed.

– Current density and pH are the main parameters that affect thegeneration of aluminum and iron inside the electrochemicalcell. When increasing current density, aluminum and iron con-centration increase linearly. However, for aluminum electrodesthe metal dissolution efficiency decreases for very high currentdensity values.

– The main difference for both metals production is on the differ-ent behavior of the corrosion of the electrodes with pH. AlkalinepHs increases the dissolution rate of aluminum electrodes. So,in the cell cathode, in which high pHs are achieved, the corro-sion of the aluminum sheets will be favored, increasing alumi-num concentration inside the cell. Contrary, acidic pHsincreases corrosion rate of iron sheets, but its effect is not verysignificant.

– The type of coagulant species produced by electro-dissolutionof aluminum or iron sheets depends on the pH. The main spe-cies produced for both metals are amorphous hydroxides. How-ever, the electro-dissolution of aluminum deals to significantconcentrations of monomeric species for acidic and alkalinepHs, and polymeric species for pHs lower than 6. On the con-trary, for iron, monomeric species are only obtained for pHsbelow 7.

– Superficially charged precipitates are obtained for both metalsas a consequence of the adsorption of soluble charged specieson the amorphous precipitates. However, when using iron,charged precipitates are only obtained for pHs below 9, as forhigher pHs there are almost no soluble species to be adsorbedon the precipitate.

Acknowledgement

The authors acknowledge funding support from the nationalSpanish Ministry of Education and Science (Project CTM2010-18833).

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